John Goerzen
I've worked with languages with object-oriented features for awhile now. Python and OCaml, the two with which I work the most, both have OO.
One of my first projects in Haskell would be to write a Haskell version of Python's ConfigParser[1] class. I wrote[2],[3] a version of this for OCaml that works very well.
In a nutshell, ConfigParser is a utility for working with sectioned configuration files in a style similar to the familiar .ini files from Windows. It has methods to read a configuration file, get/set the items that are being configured, and write a new file back out. This, then, is a fairly typical metaphor for OO programs: an instance of a class has some state that can be accessed or modified, and possibly stored and retrieved.
So I am thinking about a ConfigParser for Haskell. The first thing that occured to me is that Haskell has no OO features, so I'm not sure what is the best way to handle the "class" and its various methods.
I would expect these signatures: | module Configuration where | | type Key = String | type Value = String | type Configuration = FiniteMap Key Value | | read :: FileName -> IO Configuration | write :: FileName -> Configuration -> IO () | update :: Configuration -> Key -> Value -> IO () | lookup :: Configuration -> Key -> Value
The next thing that occured to me is that, unlike OCaml and Python classes, Haskell has no mutable variables. A call like config.setOption("main", "initpath", "/usr") in Python -- which alters the state of the config object and returns nothing -- would be impossible in Haskell (unless perhaps the FiniteMaps are mutable somehow?)
The purest approach is to have every configurable function accept a Configuration. Functions that in Python would alter the dictionary would simply return a new one.